v2: acme-dns-tiny: if a CNAME is defined on _acme-challenge.example.org following it to decide where to install the TXT records
Limitation: it only follows one CNAME for the moment, maybe we should
go through CNAME chain, but it will requires to detect CNAME loop too.

The new option TTL is used to configure the TXT resource records installed
by acme-dns-tiny: this allows us to correctly define records according
to the DNS server policy.
It allows too to fix issues about invalid TXT record errors, caused by
the ACME server cache which hidden new TXT records to the ACME server.